1. Field of the Invention
The invention relates to a magnetic recording system suitable for high density recording and, more particularly, to a magnetic recording system having a recording density of 10 Gigabits or more per square inch.
2. Description of the Related Art
The realization of a large capacity is required more and more in a magnetic disk apparatus serving as an external magnetic recording system of a computer. To realize the large capacity, in a magnetic head of the magnetic recording system, a recording unit and a reproducing unit are separated, an electromagnetic induction type magnetic head is used in the recording unit, a magneto-resistance effect type head is used in the reproducing unit, and a combination head in which these two heads are combined is used. According to the magneto-resistance effect type head, since a reproducing sensitivity is higher than that of the conventional electromagnetic induction type head, a recording bit becomes fine, and even if a leakage flux decreases, a sufficiently high reproduction output can be obtained. The development of a giant magneto-resistance effect type head of a spin valve type having an even higher reproducing sensitivity is also progressing. A magnetic recording medium is constructed by a Co alloy magnetic layer made of CoCrTa, CoCrPt, or the like, and a Cr underlayer to control crystal orientation of the magnetic layer. The Co alloy magnetic layer has a hexagonal close packed (hcp) lattice structure in which a c axis is used as an axis of easy magnetization, and it is considered to be desirable that the direction of the axis of easy magnetization is isotropically oriented in the plane of the magnetic recording medium as for an in-plane magnetic recording medium, and methods of improving the orientation of the direction of the easy axis of magnetization in the plane of the magnetic recording medium (JP-A-62-257618, JP-A-63-197018) have been proposed. In the case of using the magneto-resistance effect type head as a reproducing head, the further reduction of noises than the conventional ones is required for the medium in order to reproduce not only a signal of the medium but also the noises at a high sensitivity. The medium noises are mainly caused by a disturbance of magnetization in a magnetization transition region between the recording bits, and the narrowing of such a region contributes to the reduction of the medium noises. For this purpose, it is effective to make magnetic particles of the magnetic film of the medium fine. When the magnetic particles are made fine, however, the magnetization thermally fluctuates, and the recorded magnetization is attenuated with the elapse of time. Generally, it is known that as a value Kuxc2x7V/kxc2x7T obtained by dividing the product of a magnetic anisotropy constant Ku and a volume V of a particle by the product of the Boltzmann constant k and a temperature T decreases, thermal instability increases (P. Lu et al., xe2x80x9cThermal instability at 10 Gbit/in2 magnetic recordingxe2x80x9d, IEEE Transactions on Magnetics, Vol. 30, No. 6, Nov. 1994, pp. 4230-4232). Although it is accordingly desirable to use a material having a large Ku to obtain thermal stability, in the conventional medium, the larger the value of Ku is, the more a magnetic anisotropy magnetic field Hk increases, so that a coercive force Hc of the medium also increases. Generally, however, it is known that, as a magnetic field of the head upon recording, a magnetic field which is 1.5 to 2 times as high as Hc is needed at the center of the film thickness of the medium. According to the ability of the current magnetic head, as the coercive force Hc of the medium increases, recording becomes impossible. It is therefore necessary to realize a medium such that Ku is large so that thermal stability is obtained, Hc according to the ability of the recording head is obtained, and a high output signal to noise ratio (S/N ratio) is obtained even in high density recording.
To realize a magnetic recording system suitable for high density magnetic recording, the realization of fine magnetic particles of the magnetic layer of the medium is necessary to reduce noises. An influence by a thermal fluctuation in association with it, however, causes a problem that magnetization is attenuated with the elapse of time. It is sufficient to select a medium material of a large anisotropy constant Ku as one of means for suppressing the influence by the thermal fluctuation. In the conventional technique, the material having large Ku simultaneously shows a high coercive force Hc. However, although an enough large head magnetic field is necessary to record to the medium of high Ku (high Hc), it is becoming difficult to generate an enough magnetic field according to the ability of the current head. According to the invention, even if Ku is increased, Hc can be reduced, so that the thermal fluctuation can be suppressed and a further high signal output to noise ratio (S/N ratio) can be maintained. Particularly, a magnetic recording system suitable to accomplish the recording density of 10 Gigabits or more (magnetization transition length is equal to 70 nm or less) per square inch can be provided.
According to the invention, there is provided a magnetic recording system comprising a magnetic recording medium having a magnetic layer formed on a substrate through an underlayer; a driving unit for driving the medium in the recording direction; a magnetic head constructed by a recording unit and a reproducing unit; means for moving the magnetic head relative to the magnetic recording medium; and recording and reproduction signal processing means for performing a signal input to the magnetic head and an output signal reproduction from the magnetic head, wherein the reproducing unit of the magnetic head is constructed by a magneto-resistance effect type head and has a recording magnetization pattern in which a magnetization transition length of the magnetic recording medium is equal to 70 nm or less, the direction of an axis of easy magnetization of the magnetic particles in the magnetic layer of the magnetic recording medium is three-dimensionally distributed (an axis of easy magnetization is inclined in the film thickness direction relative to the film surface), and a squareness ratio (Mr/Ms) which is a ratio of a remanent magnetization Mr to a saturation magnetization Ms which are measured by applying the maximum magnetic field of the magnetic head in the direction of relative movement between the magnetic head and the magnetic recording medium is set to a value within a range from 0.5 to 0.6, thereby enabling Hc to be reduced even if Ku is increased, so that a thermal fluctuation is suppressed and a higher recording density can be achieved. The reason why Hc can be reduced even if Ku is increased is as follows. Generally, when an angle which is formed by the direction of the axis of easy magnetization and the head magnetic field direction is equal to 45xc2x0 or less, as such an angle increases, a magnetization reversal occurs in a weak magnetic field. By three-dimensionally distributing the direction of the axis of easy magnetization, the average angle between the direction of the head magnetic field and the direction of the axis of easy magnetization increases. Therefore, even if Ku is large, the increase in Hc can be suppressed as compared with a medium in which the direction of the axis of easy magnetization is two-dimensionally distributed. A degree of the orientation of the axis of easy magnetization is reflected in Mr/Ms, and when Mr/Ms is equal to 0.5, this corresponds to a state in which the direction of the axis of easy magnetization is perfectly three-dimensionally distributed. In the present invention, a range of 0.5 to 0.6 as a value of Mr/Ms corresponds to a region having an orientation that is slightly dominant in the in-plane direction as compared to the perfect three-dimensional random orientation.